Fresh-water production apparatus including a semipermeable membrane module for use in seawater desalination are being increasingly used in various fields because the apparatus have the merits of energy saving and space saving. Examples thereof include application to water purification processes for producing industrial water or tap water from river water, ground water, or water obtained by treating sewage or wastewater and application to semipermeable-membrane treatment steps for the reuse of sewage or wastewater and to semipermeable-membrane treatment steps for seawater desalination. The semipermeable-membrane separation device for use in seawater desalination, etc. basically has a configuration in which pretreated water which has undergone a pretreatment such as, for example, removal of suspended components is pressurized to a given pressure with a high-pressure pump and supplied to a semipermeable-membrane module and dissolved components of the liquid being treated are separated by the reverse osmosis function of the semipermeable-membrane module to obtain a permeate.
Examples of the pretreatment include sand filtration and membrane filtration, and it is especially preferred to use membrane filtration. As a pretreatment mechanism, use is made of a pretreatment membrane module including microfiltration membranes or ultrafiltration membranes. As shown in FIG. 13, a method has hitherto been used in which raw water retained in a raw-water tank 3 is supplied to a pretreatment mechanism 1 with a raw-water feed pump 4 and the pretreated water obtained is temporarily retained in an intermediate tank 18 and is then supplied with a high-pressure pump 6 to the semipermeable-membrane module 7 of a desalting mechanism 2 and treated therewith. This method, however, has a drawback in that microorganisms grow in the intermediate tank 18 and the semipermeable-membrane module 7 is prone to be fouled thereby. In addition, there are cases where the disposition of the intermediate tank 18 makes the suction pressure of the high-pressure pump 6 insufficient and this necessitates disposition of a booster pump 19 on the feed side of the high-pressure pump 6, resulting in an excess equipment cost.
In order to overcome such problems, techniques have been proposed in which pretreated water is supplied from the pretreatment modules directly to a desalting mechanism and treated (see Patent Documents 1, 2, and 3). A feature of these proposals resides in that there is no need of disposing the intermediate tank 18 which has been necessary so far.
Meanwhile, in cases when pretreatment membrane modules are continuously used for filtration, the substances to be removed, such as suspended matter, organic substances, and inorganic substances, that are contained in the raw water accumulate in the pretreatment membrane modules to increase the filtration resistance thereof, finally making it impossible to continue the filtration. For inhibiting such increase in the filtration resistance of pretreatment membrane modules, it is necessary to conduct, for example, physical washing such as back-pressure washing in which membrane-filtrated water or clarified water is forced by pressure into the modules in the direction reverse to the filtration direction to remove the fouling components accumulated on the membrane surfaces and in membrane pores, air scrubbing in which bubbles are introduced to the raw-water side of the membranes (raw-water side) to vibrate the membranes and bring the membranes into contact with one another, thereby scraping off adherent substances from the membrane surfaces, or flushing in which raw water is introduced to the raw-water side of the membranes without performing filtration, thereby removing fouling substances.
Furthermore, since polluted water remains in the raw-water-side portions of the pretreatment membrane modules after such washing, a general procedure is to discharge the water remaining in the pretreatment membrane modules and newly supply raw water. (Hereinafter, the series of operations is often referred to collectively as “washing step”.) During this washing step, one line or limited lines of the multiple lines of the pretreatment mechanism are kept isolated from the filtration lines.
In the case where there is no intermediate tank, the pretreatment mechanism is directly connected to the desalting mechanism. Because of this, there is a concern that when the pretreatment membrane modules shift from a filtration step to a washing step or shift from a washing step to a filtration step, the pressure at which the pretreated water is forced into the desalting mechanism (pressure of the pretreated water) might fluctuate to impair stable operation of the desalting mechanism and cause damage to the pretreatment modules and the pump.
In order to overcome such a problem, a fresh-water production apparatus has been proposed in Patent Document 3, the fresh-water production apparatus including, as shown in FIG. 14, a pretreatment mechanism 1 provided with a pressure control valve CV1 which serves to keep the pressure of the pretreated water constant and flow rate control valves CV2 for controlling the flow rate of raw water being supplied to pretreatment membrane modules 5.